1. (Java)
@j5ik2o

2. @j5ik2o
Scala
6
5 Java
Scala
DDD( )
(4,5,6 )

3. Singleton )
1
GoF

4. new
Hoge a = new Hoge(); // 1
Hoge b = new Hoge(); // 2 NG
1

5. public class Cache {
//
public static final Cache INSTANCE = new
Cache();
private Cache() { } // new
public void put(String key, Object value){ ... }
public Object get(String key) { ... }
}
Cache.INSTANCE.put(key, value)
Object value = Cache.INSTANCE.get(key)

6. public class Cache {
private static final Cache INSTANCE = new Cache();
private Cache() { }
public static Cache getInstance() {
return INSTNACE;
}
public void put(String key, Object value){ ... }
public Object get(String key) { ... }
}
Cache.getInstance().put(key, value)
Object value = Cache.getInstance().get(key)

7. public class Cache {
private static ThreadLocal<Cache> threadLocal =
new ThreadLocal<Cache>() {
@Override protected Cache initialValue() {
return new Cache();
}
};
public static Cache getInstnace() {
return threadLocal.get();
}
public Object get(String key) { /* ... */ }
public void put(String key, Object value) { /* ... */
}
}

8. 1:
Effective Java 3
public class CacheClient {
//
public void process() { getCache().put(...); // ... }
// Cache
Cache getCache() {
return Cache.getInstance();
// ↑
// private
// new Cache() { ... }
// ...
}
}

9. public class SingletonCache implements Cache {
private static SingletonCache instance;
public static SingletonCache getInstnace() {
return instance;
}
@Override public Object get(String key) { /* ... */ }
@Override public void put(String key, Object value) { /* ... */ }
}
public class CacheClient {
// ...
Cache getCache() {
return new Cache(){ /* */ };
}
}

10. 2:
P40-41
singleton

11. 3:
readObject
w( o )w!!!
Serializable
transient
...
readResolve
(GC)
...
private Object readResolve() throws
ObjectStreamException {
return INSTANCE; //
}

12. enum
public enum Cache {
INSTANCE;
public vod put(String key, Object
value) { /* ... */ }
public Object get(String key) {/* ...
*/ }
}
Cache.INSTANCE.put(key, value);

13. enum Jad ...
public final class Cache extends Enum<Cache>
{
public static final EnumSingleton INSTANCE;
private static final Cache $VALUES[];
static { INSTANCE = new
EnumSingleton("INSTANCE", 0);
$VALUES = (new Cache[] { INSTANCE }); }
private Cache(String s, int i){ super(s, i); }
// values, valueOf ...
}

15. (´ ∀ )
public class Singleton {
private static Singleton instance;
public synchronized static Singleton
getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}

16. ( д ) ?!
public class Singleton {
//
private static class InstanceHolder {
private static final Singleton INSTANCE = new
Singleton();
}
public static Singleton getInstance() {
return InstanceHolder.INSTANCE;
}
}

17. ..(llil!´Д`lil)n{…??
public class Singleton {
private static volatile Singleton instance
public static Singleton getInstance() {
Singleton result = instance; // (1)
if (result == null) { // (2) =>
synchronized (Singleton.class) { // (3) :
result = instance; // (4)
if (result == null) { // (5)
result = instance = new Singleton(); // (6)
}
} // :
}
return result;
}

18. getInstnace
” ”

19. Scala object
object Cache {
def put(key:String, value:Any):Unit
= ...
def get(key:String):Any = ...
}
Cache.put(key, value)
val value = Cache.get(key)

20. object Singleton
public final class Cache$ implements ScalaObject
{
public static final Cache$ MODULE$ = new
Cache$();
private Cache$() { /* ... */ }
public void put(String key, Object value) { /* ...
*/ }
public Object get(String key) { /* ... */ }
}
Cache$.MODULE$.put(key, value)
Object value = Cache$.MODULE$.get(key)